Effect Of Low Concentration Of Transition Metal Doping Upon The Reaction Performance And Mechanism Of Fe-based Oxygen Carriers In Chemical Looping Combustion

Chemical looping combustion(CLC)is a new energy conversion technology with the advantage of low cost CO2 capture.One of the cores of CLC is the design and preparation of efficient oxygen carriers.Iron-based oxygen carriers have the advantages of wide source,low cost,environmental friendliness and thermodynamic stability,but the reactivity is relatively low.The doping of transition metals can achieve the modulating effect of reactivity through synergistic effect,which is important for the enhancement of chemical looping combustion process.In this paper,low concentration Mn/Co/Ni/Cu/Zn series transition metal doped Fe-based oxygen carriers were prepared,and the study on the reaction performance and oxygen migration of Fe-based oxygen carriers was carried out.The characterization of the oxygen carrier materials,as well as the evaluation of H2-TPR and TGA performance were mainly carried out to analyze the reaction mechanism and behavior.The intrinsic reaction mechanism was investigated based on DFT calculations.The paper combines experimental and theoretical calculations to study the reaction mechanism and performance tuning of iron-based oxygen carrier CLC process from microscopic scale.The main results are as follows:1.The interactions of H2,CO,and CH4 molecules adsorbed on the surface of Fe2O3 oxygen carrier during the chemical looping process were investigated using DFT calculation method.The results of adsorption energy and approximate gradient model(RDG)analysis show that the fuel molecules interact with the Fe2O3 surface non-chemically bonded;based on the RDG plots it is visually demonstrated that the fuel molecules are mainly van der waals interactions on the oxygen carrier surface.Five transition metal doping found that Cu/Ni/Co doping is more favorable for fuel molecule adsorption on the iron-based oxygen carrier surface.Energy decomposition(EDA)quantitative analysis of the interactions revealed that physical adsorption is mainly dispersion and electrostatic interactions,and chemisorption is mainly orbital and electrostatic interactions.Among them,the bubbly repulsion of chemisorption is 1 order of magnitude larger than that of physical adsorption.The electron transfer between the adsorbed molecule and the iron-based oxygen carrier was quantified using differential charge density and Mulliken charge characterization,and the effect of transition metal doping on the iron-based oxygen carrier was explained at the electronic level by means of density of states and d-band center theory as well as work function.Cu/Ni/Co doping was found to be more effective.2.Based on Reactive Force Field Molecular Dynamics(ReaxFF MD)simulations,the effect of iron-based oxygen carriers in the CLC process fuel reactor on the cracking combustion reaction process of aromatic fragments was investigated.A very large iron-based oxygen carrier model with 16,000 atoms was established to study the combustion simulation of 16 PAHs under the action of iron-based oxygen carriers.The focus is on the distribution and changes of reactants,intermediates and products during the reaction process.Visual snapshots and radial distribution functions(RDF)were used to explain the reaction going through four stages.The RDF was used to study the reaction process of Fe...O changes during the reaction were studied,and the Fe2O3 transformation during the reaction was studied,which was largely consistent with the experimental study mainly for Fe2O3-Fe3O4 transformation.The mechanism of the generation of typical products(CO2/CO/H2O)is investigated and a complex reaction network is established.Among them,the lattice oxygen release paths were traced and statistically explained the mechanism of lattice oxygen release from oxygen carriers,and the corresponding physical model was established.It further provides a theoretical basis for the regulation of gas product species in the reactor.3.The reaction mechanism and states of Co/Cu/Mn/Ni/Zn doped iron-based oxygen carriers were investigated experimentally by using H2 and Fe2O3 as probes and DFT calculations.Firstly,the stable configuration of H2/CO/CH4 adsorption on the surface of Fe2O3,Fe3O4 and FeO was investigated and the energy decomposition was carried out for qualitative analysis,and it was found that the main effects were dispersion and electrostatic effects.The reaction energy barriers obtained from DFT calculations and the results of fitting the apparent activation energy to the nuclear contraction model(sphere)from TGA experiments showed that the order of activity of the oxygen carriers was:Cu doped>Ni doped>Co doping>Mn doping>Zn doping>pure Fe2O3,which is consistent with the results of H2-TPR experiments.The reaction can be divided into three stages Fe2O3-Fe3O4,Fe3O4-FeO,and FeO-Fe by TGA experiments.Fitting the apparent activation energy in stages reveals that E3<E2<E1.The pattern of reaction energy barriers of H2 in Fe2O3,Fe3O4,and FeO by DFT calculation is consistent.The effects of doping five transition metals on iron-based oxygen carriers were explained by XRD,BET,and in situ DRIFTS from different aspects.Cu/Ni/Co doping was found to be the best effect on the iron base.Finally,theoretical calculations and experimental results were combined to establish the particle model of the reaction and the oxygen migration path.